1. Field of the Invention
The present invention relates to a catalytic distillation process for producing tertiary alkyl ethers wherein the unreacted raffinate is substantially free of butadiene and more attractive for use in cold acid alkylation processes and other processes.
2. Related Information
The production of tertiary alkyl ethers by the reaction of primary alcohol and an isoolefin is well known in the art. The use of a distillation column reactor to simultaneously react and distill the product from the reactants has been found to be especially beneficial in this normally equilibrium limited reaction. The description of the process using the distillation column reactor and variations thereon are disclosed in commonly assigned U.S. Pat. Nos. 4,218,011; 4,232,177; 4,305,254; 4,504,687; 4,978,807; 5,118,873; 5,120,403; 5,248,836; 5,248,837; and 5,313,005. Catalytic distillation has been widely applied to etherification of isoolefins, which is described in several of the noted patents.
U.S. Pat. No. 5,431,888 discloses a multi-purpose distillation column reactor wherein a hydrogenation catalyst for hydrotreating an isoolefin containing light naphtha from a fluid catalytic cracking unit to remove diolefins and mercaptans is stacked below an etherification catalyst.
Normally the olefin feed to a methyl tertiary butyl ether (MTBE) process is a mixed C.sub.4 stream containing normal and iso butanes, normal and iso butenes and some butadiene. The isobutene (iC.sub.4.sup.=) preferentially react with methanol to form the MTBE with the remainder being essentially inert. The unreacted C.sub.4 's are frequently used as feed stock to a cold acid alkylation process which reacts the normal butenes with isobutane to form isooctane. While butadiene at the levels used does not affect the etherification, any butadiene in the feed to the alkylation process reacts to form a sludge and results in unwanted "acid consumption". It would thus be highly desirable that raffinate from a iC.sub.4.sup.= etherification be substantially free of butadiene. Also, skeletal isomerization of normal C.sub.4 olefins to isobutene normally requires removal of butadiene.
The particular feature of the present invention is the discovery that the presence of methanol in the hydrogenation zone does not adversely affect the hydrogenation of the dienes and sequence of catalysts.